Graphs of this flu season, for the data-curious

The media coverage of the flu scared me into getting a vaccine. I’ll be honest–I forgot to get one, and I’m an irresponsible young adult who kept putting it off. But right after one of my friends fell alarmingly ill, NPR ran a story about the different strains of colds and flu swarming in the cold winter air…and I caved.

However, I got my flu shot fairly late in the season. And through the terrible logic of anecdotal evidence, I began to wonder if it was even worth it. (Bad anecdotal evidence: No one in my office has the flu.) After a few moments of thought, I scolded myself for thinking that way. I work with children, who spread illnesses like wildfire. I might get the flu any old day at work, once they started coming back to the lab. And sometimes those kids bring along their baby sisters and brothers–who, under 6 months of age, depend on the immunity of the herd (which includes me) to protect them.

After all this, I just became curious to see what the data on the flu actually look like. Were the news reports exaggerated?

Luckily for me and for everyone else interested in data, the CDC publishes plenty of information on its website about the flu. Seriously–beyond my short summary, you should check it out. My conclusion: they’re only exaggerating a little. And you should probably still get a flu shot.

In the context of previous flu seasons, this one seems moderately severe. The percentage of doctor visits for flu-like illness (that is, not laboratory confirmed but characterized by a fever, cough/sore throat) may have peaked already, though it is still far too early to call it.

Check out the orange line representing 2002-2003. It’s got two small peaks, rather than just one — so it’s possible that a worse peak will emerge later this year. You can also see that the most recent season with a high number of doctor’s visits occurred in 2009 to 2010, represented by the gray line to the far left. That was declared a pandemic. (Swine flu, remember?)

Here is the pneumonia and mortality rate from influenza for the same set of years, with a convenient indicator of that week’s epidemic threshold.

Which brings up another topic. The percentage of deaths and complications from the flu is not always related to the number of doctor’s visits–rather, the severely afflicted often go straight to the hospital. Look at early 2008 in each graph. In 2008 there were about as many doctor’s visits for flu as in 2013. However, a far greater percentage of people died or contracted pneumonia in 2008.

So is 2013 a truly “severe” year? I keep hearing that word on the news. As far as I can tell, there’s no technical definition of “severe”. It’s probably some subjective combination of how widespread the illness is, how many mortalities and complications it causes, and how long it lasts. In browsing this tiny bit of data, I was struck by how little all these statistics can really tell us. I can’t tell anyone on the street if they’ll get the flu, or whether they’ll be sick in bed for a few days or whether they’ll ask someone to drive them to the hospital.

There are too many factors to track to accurately predict the global, national, and even local spread of the flu. The CDC can track the number of vaccines that are administered, hospitalizations, doctor’s visits, proportion of virus types infecting people around the U.S.–and still they can’t immediately know how the viruses might be evolving, or how many healthy people rated their flu as a “4” or a “7” on the a 1 to 10 scale of severity. From a public health perspective, it’s more crucial to know how to protect the most vulnerable populations — the young, the elderly, and the immunocompromised — from dying of a preventable illness.

And that is also what everyone else should focus on. After all, the flu vaccine is known to be safe, even for pregnant women. (In fact, it reduces fetal morality rates!)

Featured image: The H1N1 influenza virus.

P.S. If you’re curious about local data, check out the bottom of this page to look for your state in the U.S.

3 Comments

I think that downturn thingie at the end is an artifact of how the data are recorded and presented. I don’t think that is a decrease in occurrence of ILI.

I don’t know what the compliance rate is on the vaccine, but last year the flu season was very mild, possibly because vaccination rates were way up (which, in turn, was because they put the vaccine out way early). I would imagine rates of vaccination lag but correlated to severity of the season. If so, this could be a low year.

There may be a less than stellar match between vaccine and circulating flu strains this year too, which DOES NOT MEAN that it is not worth getting the vaccine.

Yes I agree — the data collection from different cities may not be complete, which may also make the last datapoint on the ILI graph look artificially low. I should have included that. I’ve also seen a couple other news reports saying that the season may have peaked…it’s unclear.

I thought about writing about the match between this year’s vaccine and the circulating viruses, but there’s not enough data yet to make a good post

I don’t know about vaccination rates and severity of the season. Unless they have a report, it’s unlikely I could figure it out. We probably need a regressor in the model to account for the vaccine match to the actual flu strains, for example. Not data that is easily found by browsing the CDC website!

Yeah, officially in writing I think they are not saying anything but verbally they are hinting. Also, I noticed that the waiting room at the clinic had one ILI person in it yesterday, but about a dozen 2-3 weeks ago last time I was there, so there’s one data point!